Many industries, (such as the chemical, pharmaceutical, oil refinery, power utility, and electronic) react chemicals at high temperatures. By "high temperatures" it is meant temperatures preferably greater than about 1000.degree. F. and more preferably greater than about 1400.degree. F. By "react" or "reaction" it is meant any chemical reaction which is exothermic (such as the synthesis, destruction, oxidation, or reduction of a chemical). For example, the oxidation may be to safely destroy a chemical by conversion to such reaction products as carbon dioxide and water, or to combust a chemical to generate energy. The reduction may be for example to reduce nitrogen dioxide to nitrogen. By "chemical," it is meant any reactable compound--whether a gas, liquid, or solid. The chemical may be, for example, organic material (such as any carbon containing compound), emissions or fumes containing an oxidizable or reactable compound from a chemical processing plant, fuel gas (such as methane) used to generate energy, liquid chemical waste from a chemical reaction, or chemical agent weapons or munitions (such as nerve gas, blister, or mustard agents).
One type of apparatus that can be used for the high temperature reaction of chemicals is a flameless matrix bed reactor. Flameless matrix bed reactors are described, for example, in U.S. Pat. Nos. 5,165,884 to Martin, et al., hereinafter referred to as "Martin," and 5,320,518 to Stilger, et al., hereinafter referred to as "Stilger," both of which are incorporated by reference herein in their entireties. In general, flameless matrix bed reactors operate by thermally reacting a process gas stream containing chemicals within a matrix bed of porous inert media (PIM). The matrix bed may be, for example, a random or non-random packed bed of heat resistant material (such as ceramic balls or saddles). The reaction is called flameless because the porous inert media permits the reaction to occur outside normal flammability limits of a reaction mixture containing air or oxygen.
The reaction of the process gas stream within the reactor is preferably carried out in a manner to establish and maintain a reaction wave in the matrix bed. This reaction wave is observed as a steep increase in matrix bed temperature, from ambient on the inlet side of the wave, to approximately the adiabatic reaction temperature of the mixture on the outlet side of the wave. This rapid change in temperature usually takes place over a distance of several inches in a typical reactor, with the actual distance usually dependent upon several physical and chemical properties.
For example, Martin discloses a flameless matrix bed reactor capable of forming a stabilized reaction wave. In Martin, a gas or vapor stream is fed into a matrix bed of heat resistant material contained within the reactor. The gas or vapor stream comprises at least one of the chemicals to be reacted. At least a portion of the matrix bed is initially heated to a temperature above the reaction temperature of the gas or vapor stream, to permit rapid reaction of the gas or vapor stream within the matrix bed.
Martin teaches that a reaction wave can be established in the reactor and can be observed by monitoring the temperature along the flow path of the gas or vapor stream. The reaction wave may be maintained at a relatively constant location within the bed by monitoring the temperature within the matrix bed and, in response, controlling the flow of any of the feed streams. The preferred flameless reactor in Martin establishes a reaction wave within the matrix bed that has a relatively flat cross-sectional profile perpendicular to the direction of flow. This reaction wave is characterized by a profile that has a substantially uniform temperature distribution at a given cross-sectional area perpendicular to the direction of flow. This reaction wave profile is hereinafter referred to as a "planar reaction wave." Such a planar reaction wave is shown in Comparative FIG. 1.
Referring to Comparative FIG. 1, there is shown a schematic of the internal temperature zones in a flameless matrix bed reactor (10) that contains a planar reaction wave (22). The flameless reactor includes a vessel (25), having a matrix bed of porous inert media (29). The vessel is lined with a refractory material (24). Prior to the planar reaction wave, there is typically a cool zone (27) that has a temperature below the uniform reaction temperature. After the planar reaction wave (22), there will be a hot region (26) that is typically at least above 1200.degree. F. By using temperature sensors (20), the planar reaction wave (22) may be located within the matrix and moved to a desired point by controlling the output end of a process controller (28).
A disadvantage to this planar reaction wave temperature profile is that in the cool zone (27), corrosive products or reactants (such as acid gases or their pre-cursors) can condense on the interior surfaces (23) of the vessel (25). This condensation can occur when the corrosive products or reactants migrate through the lining of refractory material (24) adjacent to the interior surfaces (23) of the vessel (25). Additionally, if the vessel is constructed of heat resistant metal alloys, and there is no internal lining of refractory material, corrosive products or reactants can still condense on the interior surfaces of the vessel in the cool zone (27). This condensation in turn can lead to corrosion of the interior surfaces of the vessel. Consequently, this condensation is problematic because, without using additional means to heat the walls in the cool zone (27), the life of the vessel is reduced and/or more expensive materials of construction may be needed to improve corrosion resistance.
In addition to potential corrosion problems, a flameless matrix bed reactor that generates a planar reaction wave is limited in its overall volumetric reaction rate because the reaction wave cross-sectional area is equal to the vessel cross-sectional area (i.e., the ratio of the reaction wave cross-sectional area to the vessel cross-sectional area is one). As a result, there is a limit to the overall volumetric flow rate of gas or vapor stream to be reacted, because increasing the volumetric flow rate of the gas or vapor stream beyond a certain maximum results in pushing the planar reaction wave out of the matrix bed, regardless of the matrix bed length. Consequently, to accommodate increased volumetric flow rates, the cross-sectional area of the matrix bed must be increased. However, this increase in area increases the cost of the reactor and may create installation problems where space is limited.
The limit to the overall volumetric flow rate in a planar reaction wave also undesirably limits the turndown ratio. "Turndown ratio" means the ratio of the maximum permitted volumetric flow rate of a process stream such that the chemicals are substantially reacted in the matrix bed, to the minimum permitted volumetric flow rate such that the reaction occurs in the matrix bed. A large turndown ratio is desirable so that a matrix bed reactor can accommodate a variety of flow rates for a given process stream.
Additionally, the planar reaction wave requires substantial monitoring of the temperature along the matrix bed if repositioning of the reaction wave is desired. This monitoring undesirably adds to the cost of the reactor and increases the complexity of the control system.
Although Martin discloses the possibility of using many types of reaction waves, Martin does not recognize methods of using certain reaction waves in overcoming the above aforementioned problems.
Stilger discloses another example of a flameless matrix bed reactor that recuperates or recovers the heat from the reaction. In Stilger, gas or vapor to be reacted enters a plenum in the reactor and is directed through the plenum into one or more feed tubes contained within the matrix bed. The gas or vapor flows through the feed tubes and exits the feed tubes into either the top portion of the matrix bed or the void space above the matrix bed. The reaction preferably occurs immediately after the gas or vapor exits the tubes. The gas or vapor is then directed through the matrix bed, in a direction opposite the flow of the gas or vapor entering the feed tubes, to the reactor outlet. The reacted gas or vapor as it passes through the matrix bed is used to deliver energy, by means of heat transfer across the walls of the feed tubes, to the gas or vapor in the feed tubes. The heat from the reacted gas or vapor may also be used to heat other systems or process fluids.
In Stilger, preferably a homogeneous well-mixed reaction zone is established in the matrix bed immediately following the exit ends of the feed tubes. This homogeneous well-mixed reaction zone is established through the configuration and sizing of the feed tubes, and heating the gas or vapor in the feed tube to a sufficient temperature such that when the gas or vapor exits the feed tube, the reaction of the gas or vapor occurs immediately. The well-mixed reaction zone is characterized in having a uniform temperature and composition at a given cross-sectional area.
The reaction zone established in Stilger also has the disadvantage of having a cool zone where the condensation of corrosive compounds can occur. This cool zone is located in the vessel where the feed tubes are located. Additionally, the overall volumetric reaction rate is limited by the cross-sectional area of the reaction zone. As a result, there is a limit to the overall volumetric flow rate of the gas or vapor because increasing the gas or vapor volumetric flow rate results in pushing the reaction zone down along the feed tubes where temperatures are too low for efficient reaction. Even though Stilger discloses the possibility of using many types of reaction waves, Stilger does not recognize methods of using certain reaction waves in overcoming the above aforementioned problems.
Other examples and variations of flameless matrix bed reactors are disclosed in U.S. Pat. Nos. 4,688,495; 4,823,711; 5,533,890; 5,601,790; 5,635,139; 5,637,283, 5,650,128; and the U.S. patent application Ser. No. 08/659,579 entitled Thermal Oxidizers with Improved Preheating Means and Processes for Operating Same, filed on Jun. 6, 1996, all of which are incorporated by reference herein in their entireties. However, these designs result in the volumetric flow rate being limited based on the reactor cross-sectional area and /or tend to be complicated in design.
It is therefore an object of the present invention to provide a simplified flameless matrix bed reactor which is capable of processing gas and liquid streams where the liquid streams can be vaporized within the reactor.
It is another object of the present invention to provide a flameless matrix bed reactor that is designed to prevent condensation of process streams on the interior surfaces of the vessel so as to allow for the use of less expensive materials of construction.
It is another object of the present invention to provide a flameless matrix bed reactor wherein the maximum volumetric flow rate may be increased for a given cross-sectional area size.
It is another object of the present invention to provide a flameless matrix bed reactor with greater turndown where the relocation and monitoring of the reaction wave is minimized.
These and other objects will be apparent from the following description, appended drawings, and from practice of the invention.